In a television receiver, RF signals provided by an RF source are received by an RF amplifier. The RF amplifier selects the RF signal corresponding to the channel selected by a user. The selected RF signal is coupled to a mixer where it is mixed with a local oscillator (LO) signal having a frequency corresponding to the selected channel to produce an intermediate frequency (IF) signal. The frequency of the LO signal is controlled so that the frequency of the picture carrier of the IF signal is at a nominal value, e.g., 45.75 MHz in the United States.
It is known to control the LO frequency using two tuning arrangements: a first for establishing the frequency of the LO signal at the nominal value for a selected channel; and a second for changing the frequency of the LO signal from the nominal value, e.g., to account for frequency offsets of the RF for the selected channel signal from its standard value. The frequency of the RF signal may be offset from its standard value, defined by broadcast specifications, when the RF signal source is other than a broadcast transmitter, such as a cable distribution network or video accessory like a video cassette recorder (VCR) or video disk player. The first arrangement may have a closed loop or frequency synthesis configuration, e.g., including a phase locked loop (PLL) or a frequency locked loop (FLL), or an open loop or voltage synthesis configuration, e.g., including a digital-to-analog converter. The second arrangement typically includes an automatic fine tuning (AFT) unit for generating an AFT signal representing the deviation of the frequency of IF picture carrier from its nominal value.
Usually the circuitry for generating an AFT signal is "analog" circuitry and includes a filter, often referred to as the "AFT tank", for generating an analog AFT signal having a level with a polarity and a magnitude which represent the sense and the magnitude of the deviation of the frequency of the IF picture carrier from its nominal value. In some tuning systems such as the one disclosed in U.S. Pat. No. 4,031,549, entitled "Television Tuning System with Provisions for Receiving RF Carrier at Nonstandard Frequency", issued to Rast, Henderson and Wine on Jul. 21, 1977, the analog AFT signal is used to directly control the LO frequency. In other tuning systems, such as disclosed in U.S. Pat. No. 4,868,892, entitled "Tuning System for Calculating the Local Oscillator Frequency from an AFT Characteristic", issued to Tults, Testin and Rumreich on Sep. 19, 1989, the analog AFT signal is converted to a digital signal (usually consisting of two bits) which is used to control a phase locked loop and thereby the LO frequency.
It is desirable to provide a "digital" AFT unit since the AFT tank circuit associated with an "analog" AFT unit requires components which cannot readily be incorporated in an integrated circuit (IC) and which may require alignment. In addition, a digital AFT unit is more compatible with a digital tuning control unit since interface circuitry for converting an analog AFT signal to a digital AFT signal is not required.
U.S. Pat. No. 4,485,404, entitled "Digital AFT which is Activated During the Vertical Retrace Intervals", issued to Tults on Nov. 27, 1984 discloses a tuning system in which a counting arrangement is used to measure the frequency of the IF picture carrier to produce a digital AFT signal. The counting arrangement is enabled to count cycles of the IF picture carrier during a measurement interval. The count accumulated during the measurement interval is evaluated to determine the frequency of the IF picture carrier.
While digital AFT units of the type described in the Tults patent do not require analog circuitry such as the AFT tank, the logic circuitry forming the digital AFT unit may be complex. It is desirable therefore to provide a digital AFT unit which is relatively simple in construction.